User interface design aims to enhance the visual, usability and technological qualities of an interface. It adds to the satisfaction of the person using a product or a service.
Six main qualities of a successful user interface:
_usefulness: are user’s needs satisfied by the interface functionality?
_learnability: how easy is it for the user to fulfill basic tasks when using the system for the first time?
_efficiency: after the user knows the interface, how fast is s/he able to accomplish the given tasks?
_ease of memorization: when the user returns to the interface after a while, how easily does s/he find the various functions again?
_reliability: is the interface conceived in such a way that the user makes as few mistakes as possible?
_user-friendliness : does the user like using the interface?
_usefulness: are user’s needs satisfied by the interface functionality?
_learnability: how easy is it for the user to fulfill basic tasks when using the system for the first time?
_efficiency: after the user knows the interface, how fast is s/he able to accomplish the given tasks?
_ease of memorization: when the user returns to the interface after a while, how easily does s/he find the various functions again?
_reliability: is the interface conceived in such a way that the user makes as few mistakes as possible?
_user-friendliness : does the user like using the interface?
Why involve a user interface design consultant?
To support user productivity of your professional applications and therefore reinforce the productivity of the whole enterprise.
To facilitate adoption of your applications.
To guarantee user satisfaction for your products and services.
To create a positive image during product/service sales demonstrations.
To increase customer loyalty.
To adapt your interfaces to evolving profiles of your users and customers.
_Besides, involving a consultant in the beginning of the development process can reduce the costs on long term. It is more rewarding to create a usable product from the start rather than improve its usability later on.
To support user productivity of your professional applications and therefore reinforce the productivity of the whole enterprise.
To facilitate adoption of your applications.
To guarantee user satisfaction for your products and services.
To create a positive image during product/service sales demonstrations.
To increase customer loyalty.
To adapt your interfaces to evolving profiles of your users and customers.
_Besides, involving a consultant in the beginning of the development process can reduce the costs on long term. It is more rewarding to create a usable product from the start rather than improve its usability later on.
Information technology and networks are transforming practically all the aspects of the world we live in. User interface design determines the quality of our experience while we use these systems and therefore determines the value of our experience. 
Gillian Crampton Smith,
director of Ivrea Interaction Design Institute (Italy).
